Generation of sinusoidal output signals having a variable output frequency responsive to an input signal having a variable input level is a well-known method of effecting digital communications. Common applications of such a communications technique are frequency shift keying (FSK), and derivatives of FSK such as minimum shift keying (MSK) and Gaussian minimum shift keying (GMSK).
Generation of such FSK signals is sometimes effected by circuitry involving analog devices, and such analog devices are generally bulkier and less stable (especially in temperature-varying environments) than digital devices.
In apparatuses generating FSK signals employing digital devices, sudden frequency shifts in response to changes in signal levels of input signals generally involve undesirable noise during frequency transitions.
It is desirable that stable, generally compact digital devices be employed in digital communications effecting FSK operations without generating disruptive noise during frequency transitions.